1. Field of the Invention
The present invention relates to a piezoelectric buzzer and, more particularly, to improvement in or relating to a piezoelectric buzzer of the type wherein electric force is applied to a tone generator having a piezoelectric element adhered to a resilient thin plate, thereby vibrating the tone generator to generate acoustic waves at a specific frequency.
2. Description of the Prior Art
Heretofore there have been proposed a variety of so-called piezoelectric buzzers which utilize such a characteristic of a piezoelectric element that when energized by electric force, it vibrates at a specific frequency. FIG. 1 illustrates the structure of a prior art example.
With such a structure, however, the conventional piezoelectric buzzer cannot fully meet a recent demand for miniaturization of electronic devices.
The piezoelectric buzzer shown in FIG. 1 comprises a flat cylindrical case 10, a cover 12, a tone generator 14 and a printed-circuit board 18 carrying electronic parts 16 such as a resistor, a transistor and so forth. The tone generator 14 has a substantially disc-shaped piezoelectric element 20 bonded to a substantially disc-shaped resilient thin plate 22 centrally thereof. The resilient thin plate 22 is, in turn, fixedly secured with an adhesive binder 24 as of silicone rubber to a circular projection 26 extending from the inside of the top panel of the case 10.
Under the tone generator 14, the printed-circuit board 18 is retained at its marginal edge to the inner wall of the case 10. Beneath the printed-circuit board 18 the cover 12 is also fitted at its marginal edge in the inner wall of the case 10.
The piezoelectric buzzer of such a structure not only involves the printed-circuit board 18 but also calls for electrically connecting electrode parts of the tone generator 14 to the printed-circuit board 18 by means of leads 28. Since the space in the case 10 is relatively small, such a wiring operation is troublesome and inevitably raises the manufacturing cost of the piezoelectric buzzer.
For the miniaturization of the above-described piezoelectric buzzer, it is considered to lay the electronic parts 16 on their sides on the printed-circuit board 18, or to replace them with a metal electrode face bonding type having no projecting lead, which has been developed in recent years, but these proposals do not fully satisfy the demand for miniaturization.
One of solutions to the above disadvantages is to mount the electronic parts directly on the tone generator. Although this appears to be a simple and effective solution, it practically involves the following problems. That is, if the electronic parts are placed on the tone generator at random, the specific frequency of the generator is unfavorably affected by those parts, resulting in generation of undesirable sound. Further, in case that the electronic parts are mounted on the piezolectric element in a random manner, those parts constitutes an electrical stress to reduce a feedback voltage so that a self-excited oscillation of the piezoelectric element becomes unstable and an efficiency of electroacoustic conversion is lowered.
The present inventors, upon a considerable research, have found that by mounting the electronic parts on nodal points where an amplitude of vibration of the tone generator is zero, influence to the specific frequency of the generator may be removed to thereby obtain acoustic waves at a desired frequency efficiently. Location of the nodal points can be computed relatively easily once the specific frequency of the generator is set. Alternatively, the nodal points may be actually located by plotting or tracing a portion where no vibration is observed while subjecting the tone generator to a free vibration. In order to utilize the above knowledge for industrial production of the piezoelectric buzzer, however, there is a difficulty to be overcome which is how to position the electronic parts on the nodal points exactly and efficiently during a process to fix those parts onto the tone generator by, for example, soldering.